Automatic gain control (AGC) for multichannel/wideband communications system

ABSTRACT

Automatic Gain Control (AGC) system for multi-channel signals attenuates an incoming multi-channel signal by providing a gain. The system further adjusts each individual channel, of the multi-channel signal, by supplying a second gain if needed. The AGC system is designed to ensure a received signal power is at an optimal level for analog to digital conversion or any other form of signal processing. The system also enables elimination of mid-packet gain adjustments.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.11/357,910, filed Feb. 17, 2006, which is a continuation of U.S.application Ser. No. 11/190,071 filed Jul. 26, 2005, which claims thebenefit of U.S. Provisional Application No. 60/591,381, filed on Jul.26, 2004. The entire teachings of the above applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Analog to digital conversion is a commonly used technique wherein acontinuous signal is converted to a digital signal for the purpose ofsignal processing. An analog to digital converter (ADC) is often usedfor such a conversion. ADCs typically have a limited number of bitsavailable, and thus a limited conversion range, to perform analog todigital conversions. Automatic gain control (AGC) is therefore used toadjust the power level of an incoming signal such that the ADC willreceive signals at a fixed level; thus, the number of bits required bythe ADC to perform conversions may be dramatically reduced. The AGCcontrols the gain of a system in order to maintain an adequateperformance over a range of input signal levels.

Gain will be discussed herein in terms of decibels (dB). A dB istypically used to describe the ratio between two measurements ofelectrical power, which may be arithmetically added and subtracted. AdBm represents an absolute unit of electrical power. A dBm may bedefined as A=10*log10(P2/(1 mW)), where A is the absolute unit of powerand P2 is a measurement of electrical power. The ratio of power may bedefined as P2/(1 mW)=10ˆ^((A/10)). For example, 1 dBm is one dB greaterthan 0 dBm, or about 1.259 mW (1.259=10^(1/10)).

A canonical form of a conventional AGC scheme in a digitalcommunications system 100, is illustrated in FIG. 1. The system 100comprises a Variable Gain Amplifier (VGA) 103, that receives an inputsignal 101. The AGC 105 receives a digital signal 106, digitized via anADC 107. The AGC 105 supplies information to the VGA 103 via a feedbackconnection 104. The information supplied by the AGC 105 is used inadjusting the gain supplied to the input signal 101. It should beappreciated that the gain adjustment affects the average total power ofthe signal and not the instantaneous power of the signal. Thus, the gainadjusted signal will still comprise its unique signal properties sinceits instantaneous power will be intact. A modem 109 is typically used todemodulate the signal in order to produce bits 113.

As discussed above, when designing a digital communication system, thedynamic range must be put into consideration. The dynamic range of theinput signal may be extremely large; 802.11 modems typically supportclose to 90 dB of dynamic range. Area and power requirements for an ADCtypically increases by four times every 6 dB. Hence, a large ADC dynamicrange is extremely expensive.

A solution for this problem, as previously mentioned, is to reduce thedynamic. range seen at the ADC by performing automatic gain control. Anideal AGC switches in the right amount of analog gain such that thesignal power at its output A, FIG. 1, is always the same, regardless ofthe input signal level. Hence, an ideal AGC completely eliminates signaldynamic range. Thus, the AGC is essential in such a system as itcontrols the gain of an incoming signal in order to bring the signal toa suitable level for conversion or any other form of signal processing.

As an example, consider a system that must receive single channelsignals from −100 dBm to −10 dBm, 90 dB of dynamic range. To accommodatethis range, a VGA is used that must be set to 0 through 90 dB of gain.Therefore, for a signal which is (−10 −X) dBm, X dB of gain is typicallyswitched into the signal. Using this technique the output always staysat −10 dBm. Otherwise, assuming 1 bit is required to convert a 6 dBanalog signal to a digital signal, a maximum of 15 bits would be neededto convert a −90 dBm signal. A conversion requiring 15 bits istechnically very difficult. Thus, if a −40 dBm signal arrives in thesystem, 30 dB of gain is added to the signal in order to obtain theoptimum value, dramatically reducing the amount of bits required for theconversion.

One way of building such an AGC is to simply cycle through all possiblegain settings, for example in 2 dB steps, and stop when the desiredsignal level is reached. One might choose to use a binary search insteadof a linear one to increase the speed of the acquisition.

SUMMARY OF THE INVENTION

A system and method for automatically providing gain adjustments to amulti-channel signal and gain adjustments to an individual channel, ofthe multi-channel signal, is discussed. The system comprises amulti-channel receiver, the receiver further comprising an outerprogrammable gain controller, controlling gain of a multi-channelsignal, and a plurality of inner programmable gain controllers, eachinner gain controller controlling gain of a respective individualchannel. The multi-channel receiver further comprises an analog todigital converter to digitize the gain controlled multi-channeledsignal, and each respective individual channel further comprises adigital filter.

The outer gain controller may receive feedback from each respectivechannel to adjust gain values and determine whether a signal is beingprocessed. The feedback may be provided by a modem or an analyzer. Theouter gain controller may supply a fixed nominal gain while anacquisition threshold power level or a high threshold power level is notexceeded. Once an acquisition threshold power level or a high thresholdpower level is exceeded, the outer gain controller adjusts the gain suchthat a total power is brought below the acquisition threshold. The innerand outer gain controllers may be either digital or analog.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a schematic of a conventional AGC scheme in a digitalcommunication system;

FIG. 2 is a depiction of a multi-channel signal;

FIG. 3 is a schematic of a multi-channel digital communication system;

FIG. 4 is a graphical depiction of a dual packet arrival example,according to the system shown in FIG. 3;

FIG. 5 is a graphical depiction of a second dual packet arrival exampleaccording to the system shown in FIG. 3;

FIG. 6 is a schematic of a multi-channel digital communication system,according to an aspect of the present invention; and

FIGS. 7 and 8 are an example depicting the function of the multi-channeldigital communication system, according to an aspect of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

Many problems arise when using prior art methods of automatic gaincontrol for multi-channel signals. Multi-channel systems, also referredto as wideband systems, simultaneously support multiple physical layerchannels. As an example, the case when such channels are frequencyseparated will be specifically discussed, but it should be appreciatedthat such separation may also be along other dimensions, for example,orthogonal signatures. It should be understood that these channelstypically overlap in time and are separable along some other dimension.

An example of a multi-channeled system is illustrated in FIG. 2. FIG. 2displays a three 802.11 g channel (1, 6, and 11) signal in a 2.4 GHz ISMband. An overall block diagram of a system, which may support such asignal, is shown in FIG. 3. An analog multi-channel input signal 301 isdigitized with the use of an ADC 303. An AGC 304, along with a VGA 306,is used to adjust the gain of the multi-channel signal. Themulti-channel signal is then filtered into individual channels 1, 6, and11, with the use of Band Pass Filters (BPF) 305, 307 and 309,respectively. Modems 0-2 demodulate the individual channels 1, 6, and 11into bits 0-2, respectively.

A problem in an integrated multi-channel system is that, while multiplechannels are received at different power levels and hence have differentoptimal gain settings, they are forced to share a common gain.Therefore, we provide a technique to resolve the inevitable conflictsthat result, such as mid-packet gain adjustments.

First, a two packet arrival scenario is presented to illustratedeficiencies of conventional AGCs in a multi-channel system. Considerthe packet arrival scenario depicted in FIG. 4, for the communicationsystem shown in FIG. 3. When packet 1 arrives at −20 dBm, the AGC willattenuate the power level of packet 1 down by 10 dB. Hence, post AGC,packet 1 will comprise a power level of −30 dBm, as is desired. Whenpacket 2 arrives with a power level of −40 dBm, the optimal gain forpacket 2 will be 10 dB but it instead sees a downward attenuation of 10dB, thus resulting in packet 2 comprising a power level of −50 dBm. Thisis 20 dB away from the desired power level. With the arrival of packet2, the AGC 304 and VGA 306 will see a slight increase to the total powerof the system from −20 dBm to −19.96 dBm at point 302, during time 1.Once the first packet ends, the total power of the system, at point 302,is significantly dropped to −40 dBm at time 2; thus the amount of gainapplied to the system must be significantly increased as packet 1 leavesthe system but while packet 2 is still being processed. Thus, packet 2will see a mid-packet gain change.

Now consider the same scenario as described above except that the secondpacket arrives at −23 dBm, as shown in FIG. 5. As was the case in theprevious example, packet 1 will receive a downward attenuation of 10 dB,resulting in its power level to be increased to −30 dBm. The arrival ofthe second packet will result in the overall total power, at point 302,being increased to about −18 dBm at time 1. Hence, in order to keep thedesired −30 dBm total power level, the AGC would have to switch thedownward attenuation of packet 2 from 10 dB to 12 dB in the middle ofpacket 1 (1). This type of mid-packet gain change could be catastrophicfor packet 1. Finally, consider the AGC behavior when the first packetends at time 2. The power at point 302 now drops by 5 dB to −23 dBm andthe AGC switches the downward attenuation from 12 dB to 7 dB. This gainchange is also catastrophic for packet 2.

A system is needed that will provide the desired gain adjustments formulti-channel signals, while minimizing mid-packet gain changes. A blockdiagram of a wideband AGC scheme, according to one embodiment of thepresent invention, is shown in FIG. 6. A multi-channel input signal 601is adjusted in gain with the use of an outer VGA 603. A common analogouter automatic gain controller (OAGC) 605 provides information to theVGA 603 used to continuously adjust the gain of the multi-channel inputsignal 601. The OAGC operates on the sum of the three channel powers andcannot, for instance, distinguish between signals traveling on differentchannels.

The functionality of OAGC may be described as a two state machine. Thefirst state of the OAGC is called the HUNT state. While the OAGC is inthe HUNT state, a fixed, nominal analog gain is applied. The OAGC staysin this state until the power in the band differs from an acquisitionthreshold, or the desired power level, and if the power level of theincoming signal is within the operating range of the OAGC. When thishappens, the OAGC adjusts the analog gain such that the total powerlevel is brought down to the level of the acquisition threshold, and theOAGC transitions to a second state, the LOCKED state. Thus, the OAGCattenuates the incoming signal.

As an example, shown in FIG. 7, an acquisition level is set to −30 dBm.Packet 1 arrives first at −10 dBm, 701, well within the headroom.Signals that are received in the headroom range are often clipped,therefore these signals must be brought down to the spare range, or therange in which the signal may be successfully decoded. Thus the OAGCwill attenuate the signal downward by 20 dB in order to bring the totalpower level of packet 1 to −30 dBm. The OAGC then transitions into aLOCKED state and will therefore supply a fixed downward attenuation of20 dB to all incoming signals until one of two events occur: (1) if thepower exceeds a high threshold, the OAGC adjusts the gain such that thepower level is brought down to the acquisition threshold and itcontinues to stay in the LOCKED state; or (2) if the power drops below alow threshold and if none of the modems are receiving a packet, the OAGCtransitions to the HUNT state. In FIG. 6, the ‘Rx in Progress’ signal,one per channel, is used to communicate whether a modem is receiving apacket.

As seen in the example provided by FIG. 7, a second packet 2 arriveswith a power level of −25 dBm, 703. With the arrival of packet 2, thesystem will see an overall power level increase from −10 dBm to −9.87dBm. Since the increase in the total power is so slight and does notexceed the high threshold, the system will remain in the LOCKED state.The OAGC will therefore supply a downward attenuation of 20 dB to bothpackets resulting in packet 2 comprising a total power level of −45 dBm.Packet 2 is now within the maximum (−20 dBm) and minimum (−70 dBm)decodable level range, but is 15 dB away from the acquisition level (−30dBm).

Upon receiving the analog gain adjustments, the multi-channel inputsignal 601 is then digitized with the use of an ADC 607. BPFs 609-611filter the multi-channel signal 601 into individual channels. In orderto fully utilize the word length of the digital signal in the individualchannels, the individualized digital signal may be further adjusted inorder to bring the signal to the acquisition power level. The gain ofthe individual channels are digitally adjusted, if needed, with the useof inner VGAs 613-615. Inner automatic gain controllers (IAGC) 617-619provide information to the inner VGAs 613-615, respectively, used toadjust the gain of the individual channels.

Functionally, inner AGCs are similar to conventional AGCs, with onedifference being that they are entirely digital (there is no analog gainto control). Each channel comprises its own IAGC which operates on theoutput of the channelizing filter. The IAGCs operate on a singleparameter, the desired reference level. When the input signal to theIAGC differs from the reference level, the digital gain is adjusted tocorrect for that difference.

In the example provided by FIG. 7, once packet 1 is filtered into itsindividual channel, the IAGC will not digitally adjust its gain sincethe power level of packet 1 is already at the acquisition level (−30dBm). Once packet 2 is filtered into its individual channel, the IAGCwill add 15 dB of gain in order to bring the power level of packet 2 tothe acquisition level. The digital adjustment of packet 2 is completelyindependent of the processing done to packet 1. Thus, each packet mayhave its gain individually adjusted, eliminating mid-packet gainadjustments.

The individual channels are then demodulated with the use of modems621-623. The modems 621-623 also provide feedback to the common OAGCidentifying if a packet is being processed. It should also beappreciated that feedback may be provided with the use of other devices,for example, an end point analyzer.

Considering the two packet scenario, depicted in FIG. 4, in relation tothe present invention, mid-packet gains are no longer an issue. Thefirst packet is handled in a similar manner with the present invention,as would be with a conventional AGC. A downward attenuation of 10 dB isapplied to the input signal comprising packet 1 (initially comprising apower level of −20 dBm); therefore, the desired power level of −30 dBmis achieved. The OAGC has therefore adjusted the analog gain such thatthe total power is brought to the acquisition threshold (−30 dBm) andwill then transition to the LOCKED state. Since the power of packet 1 isalready at its desired level, the IAGC for path 1 will not need toadjust its gain.

When packet 2 arrives, the total power level of the system will beincreased from −20 dBm to about −19.96 dBm at time 1, given that this isa minimal increase in power, it will probably not be significant enoughto cross the high threshold. Hence, the OAGC will stay in the LOCKEDstate and a downward attenuation of 10 dB will also be added to packet2. Thus, packet 2 will now comprise a power level of −50 dBm. The IAGCof the individual channel comprising packet 2, will adjust the gain andbring the −50 dBm packet up to −30 dBm by adding 20 dB of gain. Ofcourse, the ADC must have enough spare dynamic range to support thedigitization of the −50 dBm signal.

When the −20 dBm signal ends, the OAGC will notice a 20 dB drop inpower, which may take it below the low threshold. However, the modem onthe individual channel comprising packet 2, will indicate that a receiveis in progress and the OAGC will wait for that to finish beforetransitioning back to the HUNT state. Thus, as may be seen from theabove example, the OAGC acts as an attenuator and shifts the incomingsignal downward, while the IAGC supplies a gain to the individualchannels in order to raise the signal to the acquisition level.

The values of the maximum and minimum thresholds, acquisition, andmaximum and minimum decodable levels are determined by systemrequirements. The acquisition threshold may be set as in conventionalAGCs. It is simply the desired signal level one wishes to see at the ADCinput. The high threshold should be set higher than the acquisitionthreshold plus the single sided OAGC acquisition error but no higherthan the tolerable saturation limit. The low threshold should be setlower than the acquisition threshold minus the single sided AGCacquisition error. A problem in setting the low threshold too low isthat the OAGC will not unlock even after the packet that caused the AGCis finished.

Due to several noise sources that affect signal power estimation andgain control, practical AGCs always have a finite acquisition error. Soan AGC with +/−1 dB of acquisition error guarantees that the output ofthe variable gain stage will be correct to within that tolerance if theinput is within the specified dynamic range.

When selecting the value of the threshold levels, it is useful toexamine statistical data to determine the range where most of theincoming signals will fall. The solution presented is not a perfectsolution as there are occasions where a packet may be dropped orsaturated, as is shown in FIG. 8. In the example provided by FIG. 8, afirst packet 1 arrives with a power level of 0 dBm, 801, thus the OAGCwill a downward attenuation of 30 dB to packet 1 and then transitioninto the LOCKED state. The arrival of packet 2, at a power level of −25dBm, will increase the total power of the system by barely 0.01 dB, thusthe high threshold will not be exceeded, keeping the OAGC in the LOCKEDstate. Therefore, packet 2 will also receive a downward attenuation of30 dB, resulting in a power level of −55 dBm. The power level of packet2 is now below the minimum decodable level and will therefore need afurther adjustment in the individual channel with use of the IAGC.Although packet 2 is below the minimum decodable level, the IAGC willstill be able to boast the signal up the to acquisition level. Signalscoming in below the minimum decodable level will be adjusted in gain, orboasted up into the spare range, while the noise associated with thesignal will also be boosted. A modem in such a case may not have thesignal to noise (SNR) capabilities to decode the signal.

The amount of headroom budgeted for the system must also be put intoconsideration. For example, consider if 15 packets arrived at the sametime, all at the acquisition level (−30 dBm). Although no gain would beneeded, the system would see an overall power level of −18.24 dBm. Thus,the headroom must be set at a level greater than −18.24 dBm in order toaccommodate the incoming signal. For the above mentioned reasons it isalso useful to statistically examine the range incoming signals arelikely to fall.

This invention is applicable to any communications systems that supportstwo or more concurrent physical layer channels and minimizes the needfor mid-packet gain adjustments. Although the channels discussed in thisapplication are separated in frequency to illustrate the key concepts ofthe invention, it should be appreciated that the channels may beseparable along other dimensions.

Furthermore, although an analog OAGC was considered, it is conceivablethat the OAGC could be fully digital in those communications systemsthat utilize this invention to minimize the digital word-length (whichis analogous to ADC dynamic range). Therefore, all controllers describedherein may be analog or digital controllers and use proportional,integral, and differential (PID) controllers, state-space controllers,or other forms of control known in the art.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A multi-channel receiver comprising: an outer programmable gaincontroller controlling gain of a multi-channel signal; and a pluralityof inner programmable gain controllers, each inner gain controllercontrolling gain of an individual channel.
 2. The multi-channel receiverof claim 1, further comprising an analog to digital converter todigitize the gain controlled multi-channel signal and wherein eachchannel further comprises a digital filter.
 3. The multi-channelreceiver of claim 1, wherein the outer gain controller receives feedbackfrom each channel to adjust gain values.
 4. The multi-channel receiverof claim 3, wherein the feedback is provided by a modem.
 5. Themulti-channel receiver of claim 3, wherein the feedback is provided byan analyzer.
 6. The multi-channel receiver of claim 1, wherein the outergain controller supplies a fixed nominal gain while an acquisitionthreshold power level is not exceeded.
 7. The multi-channel receiver ofclaim 6, wherein once the acquisition threshold power level or a highthreshold power level is exceeded, the outer gain controller adjusts thefixed nominal gain such that a total power is brought below theacquisition threshold.
 8. The multi-channel receiver of claim 1, whereinthe inner gain controller adjusts a gain when an input signal to saidinner gain controller differs from a predetermined reference level. 9.The multi-channel receiver of claim 1, wherein the outer programmablegain controller is an analog gain controller.
 10. The multi-channelreceiver of claim 1, wherein the inner programmable gain controller is adigital gain controller.
 11. A method of automatic gain control formulti-channel communications comprising: automatically controlling gainof a multi-channel signal with an outer programmable gain controller;and automatically controlling gain of individual channels with aplurality of inner programmable gain controllers.
 12. The method ofclaim 11 further comprising: digitizing the gain controlledmulti-channel signal with an analog to digital converter, wherein eachchannel further comprises a digital filter.
 13. The method of claim 11,wherein providing an outer gain controller further comprising: receivingfeedback from each channel to adjust gain values.
 14. The method ofclaim 11 further comprising: supplying a fixed nominal gain, with theuse of the outer gain controller, to the multi-channel signal while anacquisition threshold power level is not exceeded; and adjusting thefixed nominal gain, if the acquisition threshold power level or a highthreshold power level is exceeded, such that a total power is broughtbelow the acquisition threshold.
 15. The method of claim 11 furthercomprising: adjusting a gain, with the plurality of inner gaincontrollers, when an input signal to said inner gain controller differsfrom a predetermined reference level.
 16. The method of claim 11,wherein the automatically controlling gain of a multi-channel signalfurther comprises: controlling an analog gain with the outerprogrammable gain controller.
 17. The method of claim 11, wherein theautomatically controlling gain of a individual channels furthercomprises: controlling a digital gain with the inner programmable gaincontroller.
 18. A multi-channel receiver comprising: an attenuation gaincontrol means for adjusting gain for a multi-channel signal; and aplurality of gain control means for adjusting gain of individualchannels of the multi-channel signal.
 19. A multi-channel receivercomprising: an outer variable gain amplifier that receives amulti-channel input signal; an outer analog gain controller thatcontrols the gain applied to the multi-channel input signal; an analogto digital converter that digitizes the multi-channel input signal aftergain adjustment; a plurality of band pass filters, said filtersreceiving the multi-channel signal and each filter outputting anindividual channel; a plurality of inner variable gain amplifiers, eachreceiving a respective individual channel; a plurality of inner digitalgain controllers, each controlling the gain applied to a respectiveindividual channel; and a plurality of feedback mechanisms providingfeedback from respective channels to the outer analog gain controller toadjust gain values.
 20. The multi-channel receiver of claim 19, whereinthe plurality of feedback mechanisms are modems.
 21. The multi-channelreceiver of claim 19, wherein the plurality of feedback mechanisms areend point analyzers.
 22. The multi-channel receiver of claim 19, whereinthe outer gain controller supplies a fixed nominal analog gain while anacquisition threshold power level is not exceeded.
 23. The multi-channelreceiver of claim 22, wherein once the acquisition threshold power levelor a high threshold power level is exceeded, the outer gain controlleradjusts the analog gain such that a total power is brought below theacquisition threshold.
 24. The multi-channel receiver of claim 19,wherein the inner gain controller adjusts a digital gain when an inputsignal to said inner gain controller differs from a predeterminedreference level.